1. Field of the Invention
The present invention concerns improvements of so called fluid-bed drying or cooling processes and devices.
2. History of the Related Art
If a layer of bulky product placed on an air or gas permeable grid, is crossed by the corresponding flow, then for a given air, or gas amount, representing an average velocity trough the product layer, the product particles are held in suspension with a bubbling motion, which represents the so called fluidization phenomenon.
The fluidization is used in dryers, or coolers, as with hot fluidization air moist product can be dried and with ambient or cooled fluidization air, hot product can be cooled.
The grid supporting the fluidizable product and enabling the uniform and homogeneous air or gas-flow disribution is called "bedplate", the product-layer "bed" and the whole system is a "fluid-bed".
Generally, fluid-bed dryers or coolers are "continuous", that means, the product continuously enters the system at one of its side, meanwhile the dried or cooled product, is discharged in a continuous way on the opposite side after having crossed the total leght of the bedplate, as a fluidized-bed is flowing like water.
Particular difficulties and inconveniences of the presently known fluid-bed systems are the following:
Wet product particles entering the fluid-bed dryer are often agglomerated in lumps, which cannot be dispersed and fluidized, remaining settled on the bedplate, which can produce product degradation or even fire in case of combustible products, like coal.
Particle-size range of some bulky products can sometimes represent a ratio of 1/5000.
As the fluidization velocity ensuring the suspension of a particle depends from the particle size, for a given average air or gas velocity, the heaviest particles will not be fluidized, remaining settled on the bedplate producing the above mentioned inconveniences.
If the fluidization velocity, this is the amount of air or gas, is sized for the heaviest particles, the smallest ones will no more held in suspension, but pneumatically conveyed and instantly discharged from the fluid-bed without being sufficiently dried or cooled.
For the same particle size, the fluidization velocity is higher in wet state, this is when entering the dryer, than when partially dried, as its weight decreases due to the loss of its moisture content, consequently the fluidization velocity should be adapted to the particle actual weight during the drying process, which is not the case of conventional dryers.